The grain size in aluminum castings, e.g. ingots, slabs, strips is an important industrial consideration and it is almost always advantageous to provide a high degree of grain refinement. It has thus become a common practice in recent years to add master alloys to molten aluminium in order to achieve fine, equiaxed grains after solidification which otherwise tend to be coarse and columnar. A fine, equiaxed grain structure imparts to a casting, high toughness, high yield strength, excellent formability, good surface finish and improved machinability. Furthermore, a sound grain-refining practice avoids hot tearing and porosity which can result from the occurrence of large columnar grains, allows a marked increase in casting speed and improves the homogeneity of the cast structure by refining the distribution of secondary phases. The use of grain-refining alloys in casting of ingots, billets and strip, has thus become a standard practice in aluminium foundries worldwide.
It is well known that addition of titanium to aluminum alloys causes grain refinement of the resulting castings through nucleation of alpha aluminum by the primary Al3Ti phase via the peritectic reaction. Additions of boron were shown, by the seminal work of Cibula in the late 1940's, to remarkably improve grain refinement of aluminum by titanium at hypoperitectic concentrations. As a result, Al—Ti—B master alloys emerged as potential grain refiners for aluminum alloys. At present, there is a variety of commercial grain refiners of this type as well as comprehensive literature on this system and its implications on the grain refinement. The microstructure of these alloys consists of TiB2 and Al3Ti particles in an aluminium matrix with extremely small amounts of Ti and B in solution. When Al—Ti—B master alloys are added, the aluminum matrix dissolves and these particles which subsequently act as heterogeneous nucleation sites are released into the melt. The mechanism of grain refinement by Al—Ti—B master alloys involves segregation of solute Ti onto the TiB2/melt interface accompanied by the formation of an interfacial layer which takes part in the nucleation process (Mohatny 4-7). Extensive detailed discussion on theories of grain refinement can be found in the literature (Mohatny 2-8). The use of AlTiB type master alloys for grain refinement of aluminum alloys today is an established procedure and has become widespread in the aluminum foundry industry.
Aluminum grain refiner alloys consist typically of 2-12 wt % titanium and 0.1-2 wt % boron, the balance being commercial grade aluminum with normal impurities. Examples of these alloys are disclosed in U.S. Pat. Nos. 3,785,807, 3,857,705, 4,298,408 and 3,634,075. Various methods for the production of Al—Ti—B grain refiner master alloys have been described in numerous patents (Murty 24-31) as well as in the open literature (Murty 3, 15, 23, 42-48).
The invention outlined in U.S. Pat. No. 6,228,185 teaches a process for making a castable aluminium-based matrix melt, by reacting, within an aluminium-based melt, precursor compounds, so as to produce boride ceramic particles dispersed in the melt. The preferred precursors are potassium borofluoride, KBF4, and potassium hexafluorotitanate, K2TiF6. The two salts are fed to the aluminium-based melt at a controlled rate, while maintaining stirring of the melt. Another technique reported in GB-A-2,257,985, GB-A-2,259,308 and GB-A-2,259,309, referred to as the reactive casting technique, also uses a mixture of K2TiF6 and KBF4 in contact with molten aluminium to form and disperse the TiB2 particles in the molten alloy.
While KBF4 is conventionally employed as the commercial source of boron, alternative sources for boron have also been identified. The process described in U.S. Pat. Nos. 5,415,708 and 5,484,493 involves adding a boron containing material selected from the group consisting of borax, boron oxide and boric acid and their mixtures plus K2TiF6 to a bath of molten aluminum and stirring the molten mixture to produce an aluminum base alloy consisting essentially of from 0.1 to 3.0% boron, from 1 to 10% titanium.
Sources of titanium other than K2TiF6, include titanium sponge, titanium turnings and titanium oxide. U.S. Pat. No. 3,961,995 describes a process for producing Al—Ti—B alloys by reacting liquid aluminum with titanium oxide and boron oxide in solution in molten cryolite and quenching the alloy rapidly to cool and solubilize the reaction product. Zhuxian et al (Murty: 53, 54) have prepared Al—Ti—B master alloys by the thermal reduction and electrolysis of titanium dioxide and diboride trioxide in cryolite alumina melts in the presence of aluminum at 1000C. Sivaramakrishnan et al. (Murty: 49-52) have successfully prepared Al—Ti—B master alloys by the reaction of B2O3 and TiO2 with molten aluminum. However, this method requires high operating temperatures generally in excess of 1000C. Krishnan et al (Murty: 59) have melted aluminum and titanium sponge together and allowed the melt to react with KBF4 in order to prepare Al—Ti—B master alloy.
In the process described in French Patent Specification No. 2,133,439, two aluminum masses, one containing dissolved titanium and the other dissolved boron, are contacted at elevated temperature (above 1000C.), resulting in the formation of titanium diboride crystals which are insoluble in the aluminum. The mixture then has to be intensively cooled in order to avoid growth of the TiB2 crystals, which reduces the effectiveness of the master alloy. Accordingly, mixing of the two molten masses and cooling have to be carried out at virtually the same time, which necessitates expensive apparatus, both for mixing and for cooling, so that it is only possible to use very small batches at a time.
Among the above techniques, that involving the reaction of halide salts with molten aluminium is the most popular. This technique uses low melt temperatures (750-800) compared to thermal reduction (1000C.) and utilises the exothermic nature of the reaction between the salts and the molten aluminum. Al—Ti—B grain refiner alloys according to this technique are conventionally produced batchwise in an electric induction furnace. The alloying ingredients, typically provided in the form of the double fluoride salts of titanium and boron with potassium in the required proportion are fed to a stirred body of molten aluminum in an induction furnace between 700.-800C. The salt mixture is drawn below the surface of the melt by means of an electromagnetic stirring action, and are reduced to Ti and B by Al. These complex salts react with liquid aluminium quickly and very efficiently producing a melt with dispersed particles of Al3Ti and (Al,Ti)B2 and high yields of Ti and B in the final alloy [4, 5, 7, 9]. Measures are taken to allow the reaction product, molten potassium aluminum fluoride, to rise to the surface of the melt where it forms a discrete layer which is then removed by decanting. The batch of molten alloy thus obtained may be transferred to a separate casting furnace which is typically an electric induction furnace where electro-magnetic stirring helps to keep the insoluble TiB2 particles suspended in the melt. The alloy may be cast into either an ingot for further working to rod by rolling or by extruding or directly into a rod casting machine, such as a Properzi caster.
In addition to the batch process, there are several methods to produce AlTiB grain refiners continously. Such a continous process is described in U.S. Pat. No. 5,100,618 for producing an Al—Ti—B grain refiner. U.S. Pat. No. 5,057,150 also discloses a process for the production of an Al—Ti—B grain refining rod, in which molten aluminum is continuously passed through a confined reaction zone. Titanium and boron precursor compounds, e.g. salts, are continuously added to the molten aluminum in the reaction zone and the content of the reaction zone is continuously stirred to submerge the salts within the aluminum melt. The molten alloy formed is continuously transferred via a transfer conduit from the refining zone to a casting station.
A recent work has indicated that holding the master alloy melt at approximately 750° C. for several hours after the salt reaction is complete, produces master alloys having very good grain refining properties [5, 17, 22, 23]. U.S. Pat. No. 4,612,073 discloses a new aluminum grain refiner alloy with a controlled, effective content of ‘duplex’ crystals which are claimed be extremely potent grain refining agents. The duplex crystals are made by producing aluminides that contain boron in solution, and then by aging this aluminide in a manner to precipitate at least part of the boron to form the duplex crystals.
Several patents (U.S. Pat. Nos. 3,785,807 and 3,857,705) have disclosed concepts to obtain improved grain refining alloys by controlling the morphology of the TiAl3 crystals. These disclosures are often contradictory and do not clearly solve the problems.
During the investigation of existing grain refiners and testing of various alloys and methods described in the prior art, it became apparent that two batches of the same product, apparently produced in nearly the same manner and with nearly the same bulk chemistry, behave differently when used as a grain refiner. Besides, certain difficulties are involved in processing Al—Ti—B master alloys, and the results obtained in regard to grain refinement differ very appreciably, according to the composition of the alloy and its method of preparation. This could at least in part, be due to the fact that the microstructure and the performance of a grain refiner are highly sensitive to the processing parameters used in the production of the master alloy [1, 3].